Lab-scale processing techniques are excellent for making small proof-of-concept devices, but there is often an engineering gap to make large scale devices, while maintaining device efficiency. Here we look at an exciting and relatively new solar panel material, lead halide perovskites. Lead halide perovskites, which are semiconducting solar absorber materials, have lab-scale solar cell device efficiencies of over 20% and are expected to be much cheaper than current commercial solar cells. However, the scale-up of perovskites has not been achieved to make commercial cells. Here researchers investigate various solution deposition techniques in the context of an inexpensive and scalable roll-to-roll printing method to make high efficiency perovskite solar cells, up to 19.6%.
Roll-to-roll printing is used to print newspapers worldwide. It’s an old, simple, and effective technique, where a flexible substrate is rolled past a dye printer (Figure 1a). In this case, instead of paper, the substrate is conductive flexible glass, and instead of printer ink, it’s a dye solution of methylammonium iodide and lead iodide, the precursor for the perovskite solar material. This can be used for ¼ meter scale substrates (Figure 1b), which is significantly larger than previous film deposition techniques.

To investigate making high quality large scale thin-films via roll-to-roll printing, the researchers needed to understand a few components. Thin-film growth is dictated by four processes: prenucleation (I), nucleation (II), nucleation and growth (III), and crystal growth(IV) (Figure 2). Nucleation a critical step of thin-film growth, where minor amounts of the desired product (perovskite in this case) begins to form from an evaporating solution. To rapidly nucleate the perovskite, which is required for printing techniques, a low-boiling point and fast evaporating solvent, methylamine (10% w/w) in acetonitrile was used. The acetonitrile is a low-boiling point solvent, meaning it will evaporate quickly, and the methylamine is used to solubilize the lead iodide component. This solvent also favors steps III and IV, by also inducing rapid growth in addition to nucleation. By using this system, the perovskite is approximately 60% formed within 1 s of deposition. This can be visually observed by a black film (indicative of perovskite) forming extremely rapidly. After 110 s, the perovskite is fully formed, making this solvent system an excellent choice for the fast roll-to-roll printing.

Figure 2. Right: Plot of solution concentration vs. time as a thin film is formed. Step (I) is the critical region to nucleate growth. Left: images of thin-films of perovskite showing smooth continuous films produced by roll-to-roll printing.

Utilizing this solvent system, the researchers investigated slot-die coating as a technique (Figure 3a). Slot-die is commonly accepted as a system compatible with roll-to-roll printing. The technique relies on a meniscus-guided coating. This means the liquid dye is in contact with both the slot-die and substrate through the liquid meniscus (i.e. bubble of liquid at the substrate surface) (Figure 3a). As the substrate is moved past the die-slot, the meniscus of the liquid stays with the slot-die, but a thin solution of dye is left behind. The solvent can evaporate extremely quickly from this thin solution leaving behind thin-films of perovskites (Figure 3b).

To test their slot-die technique vs. lab scale techniques, the researchers fabricated solar cells from their films. The solar cells fabricated here consists of indium zinc oxide (IZO) coated flexible glass, tin oxide, perovskite, spiro-MeOTAD, and gold (figure 3c). The IZO glass and gold are leads, just like the leads of a battery. But the IZO is transparent to allow light into the solar cell. The tin oxide and spiro-MeOTAD form n and p components. The n and p represent negatively doped and positively doped, meaning they have an intrinsic negative and positive charge. When light shines on the perovskite, electrons are excited, and are attracted to the p-type positive material. This creates the current in the solar cell. The solar cells fabricated with this method have 19.6% efficiencies, which is very high efficiency, considering how inexpensive this reel-to-reel printing technique can be compared to current commercial solar panel cost.

This method demonstrated is one step forward to producing high efficiency and scalable perovskite solar cells. The future progress will be maintained in producing higher stability solar cells, but this is a remarkable demonstration of using chemistry to bridge the gap between lab scale production and mass production.

I love designing and making things. While my interests in science are all over the place, I like chemistry because it allows me to make things on the atomic level. My interests in research are mostly energy related: Solar panels, battery technology, and electronic components. Outside of my work, I'm an avid cyclist, nature enthusiast, and at home chef.